Power converters have been extensively employed in medium voltage motor drives and other applications in which electrical power needs to be converted from DC to AC or vice versa. Such conversion apparatus is commonly referred to as an inverter for converting DC to AC, or alternatively as a rectifier if the conversion is from AC to DC power, where the AC power connection typically provides a multi-phase output or input, respectively. Multi-phase converters are often constructed using an array of high-voltage, high-speed switches, such as gate-turnoff thyristors (GTOs), insulated-gate bipolar transistors (IGBTs) or other semiconductor-based switching devices, which are selectively actuated through pulse width modulation (PWM) to couple the AC connections with one or the other of the DC bus terminals, where the timing of the array switching determines the power conversion performance. In medium voltage motor drive applications, the timed control of the switch activations in advanced inverter type power converters is used to provide variable frequency, variable amplitude multi-phase AC output power from an input DC bus, whereby driven motors can be controlled across wide voltage and speed ranges.
Neutral point clamped (NPC) converters include two similarly sized high voltage capacitors connected in series between the DC bus lines, with the capacitors being connected to one another at a converter “neutral” point node. In these NPC converters, three-level switching control is often used to provide three switching states for each AC terminal, with the AC terminal being selectively coupled to either of the DC terminals or to the neutral node. Three-level switching techniques allow higher operating voltages along with better (e.g., lower) total harmonic distortion (THD) and electromagnetic interference (EMI) than do comparable two-level inverter designs. Several PWM switching techniques have been used in high or medium voltage NPC power converters to control the switch array, wherein space vector modulation (SVM) approaches are increasingly used because of good harmonic profile, effective neutral point potential control, and ease of digital implementation. In NPC power converters, it is desirable to maintain the neutral voltage at a constant level with the two capacitor voltages being substantially equal, a goal known as neutral point balancing. Problems may arise if the voltage at the NPC inverter neutral point deviates from the mid-point of the DC bus, including stresses to components of the converter itself and/or to devices being powered by the converter, as well as adding harmonic distortion to the output of the inverter. To control the neutral point voltage, many converters are equipped with feedback control apparatus. However, such closed loop neutral balancing approaches are costly, requiring feedback sensing apparatus and advanced control algorithms to regulate the neutral voltage while also providing the desired AC output waveforms. In addition to neutral point balancing, it is desirable to minimize the operating frequencies of the array switches. These problems are of course balanced against frequency, amplitude, and other performance and control requirements for a given converter application. Thus, there remains a continuing need for improved three-level power converters as well as SVM methods and control systems for operating power converters for use in medium voltage motor drives and other applications requiring electrical power conversion.